During the subterranean drilling process, measurements of the porosity of earth formations surrounding a borehole are conventionally made to locate and characterize, for example, hydrocarbon reservoirs. These measurements can be made while the bore is being drilled using techniques known as logging-while-drilling (LWD) or measurement-while-drilling (MWD). One method for measuring porosity involves deploying a tool within a borehole with a neutron source and two neutron detectors that are spaced away from the source. Fast neutrons emitted into the formation from the neutron source tend to slow down more in formations having higher porosity. Thus, the rate of neutrons reaching the neutron detectors after traveling through a formation (“count rates”), is an indicator of a formation's porosity.
The estimation of formation porosity based on the neutron count rates can be adversely affected by borehole standoff especially in the presence of drilling fluid. Standoff refers to the space between a neutron source and the wall of borehole, which is typically filled with drilling fluid that tends to absorb fast neutrons at a rate greater than the surrounding formation. For similar reasons, effects due to particularly large borehole diameters can be problematic in estimating formation porosity.
In order to minimize standoff effects, prior art neutron detector systems commonly use mechanisms that extend to press a neutron source against the surface of a borehole; however, such methods are typically used in wire-line logging systems. The practical considerations, such as a moving drill string, encountered in LWD and MWD systems make solutions that press the neutron source against the surface of a borehole more difficult to implement. In contrast to wire-line logging, the location of a dual-neutron detector tool within a borehole during LWD and MWD operations is typically dictated by the drilling operation and thus control of the standoff is limited. In addition, LWD and MWD operations often require measurements to be taken while the drill string rotates causing the neutron source to rotate and cause a standoff that is widely variable.
Prior art systems commonly provide corrective techniques to compensate porosity estimates for the effects of standoff and borehole size. These techniques often call for ultra-sonic transducers that measure the actual standoff and bore size as measurements are made. In certain situations that can arise in LWD and MWD operations, these transducers may not always be reliable. Thick circulating drilling fluid can make it difficult for a transducer to discern the mud from the wall surface of a borehole. The inset to the transducer may intermittently become “packed-off” by becoming filled with solid material. If the drilling fluid is changed while drilling is in progress, the effectiveness of the corrective techniques that are dependent on knowing certain mud properties, may be diminished. Furthermore, the tolerance of each standoff correction is commonly a percentage of the standoff correction. Thus, in the situation where a standoff correction is substantial compared to the uncorrected porosity measurement, the certainty of the corrected porosity measurement is diminished.
There is therefore a need for reliable, inexpensive measuring tools and methods for obtaining the porosity of sections of earth formations traversing portions of a borehole that are less sensitive to the effects of borehole size and standoff. In this way, reliance on corrective techniques (and the precision of corrective techniques) for these effects would be reduced.